Sarai was the daughter of Terah by virtue of her marriage to Haran, the son of Terah. Haran was the brother of Abram, the eldest Son of Terah. When Haran died in the land of Ur, she became the wife of Abram according to the custom of the Chaldees.
This is the law of Moses:
If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her. Deut 25:5
This is the word of the LORD:
Thou shalt not uncover the nakedness of thy brother's wife: it is thy brother's nakedness. Lev 18:15
I don't know what makes you believe that Sarai was the biological sister of Abram.
Genesis 20:11
Abraham
אַבְרָהָ֔ם (’aḇ·rā·hām)
Noun - proper - masculine singular
Strong's 85: Abraham -- 'exalted father', the father of the Jewish nation
replied,
וַיֹּ֙אמֶר֙ (way·yō·mer)
Conjunctive waw | Verb - Qal - Consecutive imperfect - third person masculine singular
Strong's 559: To utter, say
“I thought to myself,
אָמַ֗רְתִּי (’ā·mar·tî)
Verb - Qal - Perfect - first person common singular
Strong's 559: To utter, say
‘Surely
רַ֚ק (raq)
Adverb
Strong's 7535: But, even, except, howbeit howsoever, at the least, nevertheless
there is no
אֵין־ (’ên-)
Adverb
Strong's 369: A non-entity, a negative particle
fear
יִרְאַ֣ת (yir·’aṯ)
Noun - feminine singular construct
Strong's 3374: Fear, reverence
of God
אֱלֹהִ֔ים (’ĕ·lō·hîm)
Noun - masculine plural
Strong's 430: gods -- the supreme God, magistrates, a superlative
in this
הַזֶּ֑ה (haz·zeh)
Article | Pronoun - masculine singular
Strong's 2088: This, that
place.
בַּמָּק֖וֹם (bam·mā·qō·wm)
Preposition-b, Article | Noun - masculine singular
Strong's 4725: A standing, a spot, a condition
They will kill me
וַהֲרָג֖וּנִי (wa·hă·rā·ḡū·nî)
Conjunctive waw | Verb - Qal - Conjunctive perfect - third person common plural | first person common singular
Strong's 2026: To smite with deadly intent
on account of
עַל־ (‘al-)
Preposition
Strong's 5921: Above, over, upon, against
my wife.’
אִשְׁתִּֽי׃ (’iš·tî)
Noun - feminine singular construct | first person common singular
Strong's 802: Woman, wife, female
Genesis 20:12
Besides,
וְגַם־ (wə·ḡam-)
Conjunctive waw | Conjunction
Strong's 1571: Assemblage, also, even, yea, though, both, and
she
הִ֔וא (hî)
Pronoun - third person feminine singular
Strong's 1931: He, self, the same, this, that, as, are
really
אָמְנָ֗ה (’ā·mə·nāh)
Adverb
Strong's 546: Verily, truly, indeed
is my sister,
אֲחֹתִ֤י (’ă·ḥō·ṯî)
Noun - feminine singular construct | first person common singular
Strong's 269: Sister -- a sister
the daughter
בַת־ (ḇaṯ-)
Noun - feminine singular construct
Strong's 1323: A daughter
of my father—
אָבִי֙ (’ā·ḇî)
Noun - masculine singular construct | first person common singular
Strong's 1: Father
though
אַ֖ךְ (’aḵ)
Adverb
Strong's 389: A particle of affirmation, surely
not
לֹ֣א (lō)
Adverb - Negative particle
Strong's 3808: Not, no
the daughter
בַת־ (ḇaṯ-)
Noun - feminine singular construct
Strong's 1323: A daughter
of my mother—
אִמִּ֑י (’im·mî)
Noun - feminine singular construct | first person common singular
Strong's 517: A mother, )
and she became
וַתְּהִי־ (wat·tə·hî-)
Conjunctive waw | Verb - Qal - Consecutive imperfect - third person feminine singular
Strong's 1961: To fall out, come to pass, become, be
my wife.
לְאִשָּֽׁה׃ (lə·’iš·šāh)
Preposition-l | Noun - feminine singular
Strong's 802: Woman, wife, female
